In recent years, surface-treated steel sheets manufactured by rustproofing steel sheets, particularly galvannealed steel sheets which are excellent in rustproofness, have been used in the fields of automobiles, household electrical appliances, and construction materials. From the standpoint of improving the mileage and crash safety of automobiles, the trend of increasing the strength and reducing the thickness of automobile body materials and thereby reducing the weight and increasing the strength of automotive bodies encourages application of high-strength steel sheets to automobiles. To increase strength, there is a direction to high-strength steel sheets that contain Si, which improves strength without reducing ductility, a low-cost strengthening element Mn, and Cr, which improves quenching hardenability. Such galvannealed steel sheets are subjected to electrodeposition coating or general painting before use. Thus, corrosion resistance after painting is important.
Conventional methods of manufacturing a hot-dip coating steel sheet based on a high-strength steel sheet will be exemplified below.
Japanese Unexamined Patent Application Publication No. 2004-323970 discloses a galvanized steel sheet, the coated layer of which is not subjected to an alloying treatment after hot-dip galvanizing. Japanese Unexamined Patent Application Publication No. 2004-315960 discloses a galvannealed steel sheet, the coated layer of which is subjected to an alloying treatment after hot-dip galvanizing. In both cases, the heating temperature in a reduction furnace is specified by a formula including the water vapor partial pressure, and the dew point and the oxygen potential are increased to internally oxidize the surface layer of the base steel. However, JP '970 and JP '960 cannot achieve an excellent characteristic of high corrosion resistance after painting.
Japanese Unexamined Patent Application Publication No. 2006-233333 discloses a technique that sets the concentration of CO2 as well as the oxidizing gases H2O and O2 to increase oxygen potential to internally oxidize a base steel surface layer immediately before coating and suppress external oxidation, thereby improving the appearance of the coating. However, CO2 may have problems in that it causes furnace pollution and carburization on the surface of steel sheets which affect the mechanical characteristics of the steel sheets. Further, no mention is made of corrosion resistance after painting.
Japanese Unexamined Patent Application Publication No. 2010-77480 A discloses a technique of modifying skin pass rolling conditions to define a flat portion. However, descriptions of corrosion resistance after painting and a surface state that is effective in improving corrosion resistance after painting are not given.
It could therefore be helpful to provide a galvannealed steel sheet having high corrosion resistance after painting.